The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the tuning of such resonators and filters.
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical/acoustic wave (produced by the RF signal) is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonant frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonant frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonant frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). The difference between these two types of devices lies mainly in their structures. An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel or shunt FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIG. 1. As shown in FIG. 1, the FBAR device 1 comprises a substrate 2, a bottom electrode 4, a piezoelectric layer 6, a top electrode 8 and a passivation layer 10. The FBAR device 1 may additionally include an acoustic mirror layer 12, which is comprised of a layer 16 of high acoustic impedance sandwiched between two layers 14 and 18 of low acoustic impedance. The mirror usually, but not always, consists of pairs of high and low impedance layers (even number of layers). Some mirrors consist of two pairs of such layers arranged in a sequence like SiO2, W, SiO2, W. Instead of the mirror, an FBAR device may additionally include one or more membrane layers of SiO2 and a sacrificial layer. The substrate 2 can be made from silicon (Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass, or ceramic materials. The bottom electrode 4 and top electrode 8 can be made of gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), aluminum (Al), titanium (Ti) or other electrically conductive materials. The piezoelectric layer 6 can be made of zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer 10 can be made of a dielectric material, such as SiO2, Si3N4 and polyimide to serve as an electrical insulator and to protect the piezoelectric layer. The low acoustic impedance layers 14 and 18 can be made of Si, SiO2, poly-silicon, Al or a polymer. The high acoustic impedance layer 16 can be made from Au, Mo or W, and in some cases, dielectric such as aluminum nitride (AlN) to make a number of layer pairs. FBAR ladder filters are typically designed so that the series resonators yield a series resonance at a frequency that is approximately equal to, or near, the desired, or designed, center frequency of the respective filters. Similarly, the shunt, or parallel, resonators yield a parallel resonance at a frequency slightly offset from the series FBAR resonance. The series resonators are usually designed to have their maximum peak in transmission at the center frequency, so signals can be transmitted through the series resonators. In contrast, the shunt resonators are designed to have their minimum in transmission, so signals are not shorted to ground. FBARs yield parallel resonance and series resonance at frequencies that differ by an amount that is a function of a piezoelectric coefficient of the piezoelectric materials used to fabricate the devices, in addition to other factors such as the types of layers and other materials employed within in the device. In particular, FBAR ladder filters yield passbands having bandwidths that are a function of, for example, the types of materials used to form the piezoelectric layers of the resonators and the thickness of various layers in the device.
The difference in the thickness in various layers in the device can be achieved during the fabrication of the device. Presently, FBARs are fabricated on a glass substrate or a silicon wafer. The various layers in the FBAR-based device are sequentially formed by thin-film deposition. In an FBAR-based device, the resonant frequency of the device usually has to be controlled to within a 0.2-0.5% tolerance. This means that the thickness of each layer in the device must be controlled in the same way. It is known that, however, the deposition of thin-film layers is difficult to control to yield a thickness within such tolerance when the area of substrate or wafer is large. For that reason, manufacturers of FBAR-based devices use wafers of 4-inches or less in diameter for device fabrication. With a small wafer or substrate, certain thickness non-uniformity can be accepted without losing many components due to the operation frequency being out of specification. However, fabricating devices on small wafers or substrates is less cost-effective than doing the same on large substrates. In the case of using large substrates, the problem associated with thickness non-uniformity becomes acute.
Thus, it is advantageous and desirable to provide a method and system to solve the problem associated with thickness non-uniformity in the fabrication of FBAR-based devices on large substrates or wafers.
It is a primary object of the present invention to provide a method and system for achieving the desired resonant frequency of the device within a certain tolerance. This object can be achieved by correcting for the thickness non-uniformity of the devices fabricated on large substrates. The thickness variations can be corrected by selectively removing material from the surface area of a wafer (with one or more layers of the device already deposited thereon), or die, before the wafer is cut into individual chips. In that context, the bulk acoustic wave device, as described herein, refers to the entire wafer or substrate that has one or more layers deposited thereon to form one or more individual chips, or part of such wafer or substrate. Moreover, the bulk acoustic wave devices referred to herein include bulk acoustic wave resonators, stacked crystal filters, any combination of the resonators and filters, and the structural variations of the resonators and filters. Furthermore, although one or more layers are already formed on the substrate, the device may or may not have all the necessary layers or the patterns of the layers, when the thickness of the device is adjusted. For example, the topmost layer on the substrate may be the piezoelectric layer, the top electrode or another layer.
Thus, according to the first aspect of the present invention, a method of tuning a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the device has a top surface layer made of a surface material and a surface layer thickness with a thickness non-uniformity profile across the top surface layer, and wherein the device has an operating frequency which varies partly with the surface layer thickness and the operating frequency can be adjusted by adjusting the thickness of the top surface layer. The method comprises the steps of:
providing, on top of the top surface layer, a mask made from a mask material having a further non-uniformity profile partly based on the thickness non-uniformity of the top surface layer;
providing an etching agent over the mask for removing the mask material from at least one mask area to expose a corresponding surface area of the top surface layer to the etching agent; and
removing part the of surface material from the top surface layer at the exposed surface area until a desired thickness of the top surface layer is reached, while simultaneously removing the mask material to modify the exposed surface area.
The top surface layer of the device can be any one of the acoustic wave generating and controlling layers and one or more layers can be fabricated on top of the top surface after the desired thickness of the top surface layer is achieved. Thus, the top surface layer can be a top electrode layer, a piezoelectric layer or a passivation layer.
Preferably, the mask material is a photoresist material, and the further non-uniformity of the mask is achieved by controlling the exposure depth of the photoresist material to a light beam, wherein the exposure depth varies based partly on the thickness non-uniformity of the top surface layer.
It is possible to control the intensity profile of the light beam to achieve the controlled exposure depth.
It is possible to introduce a pigment into the photoresist material to attenuate the light beam for controlling the exposure depth.
It is possible that the mask material is a dielectric material, and the further non-uniformity of the mask is achieved by trimming the dielectric material in a laser ablation process.
It is possible that the thickness non-uniformity of the top surface layer comprises a plurality of locations at which the top surface layer requires thickness adjustment, and the mask has a plurality of sections corresponding to the plurality of locations of the top surface layer, and wherein the mask material is selectively removed at the plurality of sections to provide the further non-uniformity profile of the mask.
Preferably, the mask material is a dielectric material, and a laser beam is used to selectively remove the plurality of sections of the mask.
Preferably, the etching agent comprises an ion beam for use in an ion beam etching process.
It is possible that the etching agent comprises a reactive ion beam for use in a reactive ion etching process.
It is also possible that the etching agent comprises an ion beam and one or more chemical agents for use in a chemically assisted ion beam etching process.
It is preferable to map the thickness of the device to determine the thickness non-uniformity profile of the device surface prior to providing the mask on top of the top surface layer, wherein the mapping can be carried out with a thickness profile measurement device or a resonant frequency measurement device.
According to the second aspect of the invention, a system for tuning a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the device has a top surface layer made of a surface material and a thickness with a thickness non-uniformity profile across the top surface layer, and wherein the bulk acoustic wave device has an operating frequency, which varies partly with a thickness of the top surface layer, and the operating frequency can be adjusted by adjusting the thickness of the top surface layer through a mask made of a mask material provided on top surface layer. The system comprises:
means, positioned above the device, for removing the mask material for providing a further non-uniformity profile of the mask partly based on the thickness non-uniformity of the top surface layer; and
means, positioned above the mask, for providing an etching agent over the mask for removing the mask material from at least one mask area to expose a corresponding surface area of the top surface layer to the etching agent, and for removing part of the surface material from the top surface layer at the exposed surface area until a desired thickness of the top surface layer is reached, while simultaneously removing the mask material to modify the exposed surface area.
Preferably, the mask material is a photoresist material and the system further comprises a light source to expose the photoresist material, wherein the light source has a non-uniformity intensity profile to control the exposure depth of the photoresist material across the device surface.
It is possible that the mask material is a dielectric material and the system further comprises a laser light source for trimming the dielectric material, and means for varying the light beam intensity across the device surface.
It is also possible to move the laser light source in a lateral direction relative to the device surface to trim the dielectric material one spot at a time.
It is also possible that the thickness non-uniformity profile of the top surface layer comprises a plurality of locations at which the top surface layer requires thickness adjustment, and the mask comprises a plurality of sections corresponding to the plurality of locations, and the laser light source selectively removes the dielectric material at the plurality of sections of the mask for providing the further non-uniformity profile.
It is possible that the mask material is a dielectric material and the system further comprises a particle beam for removing the dielectric material according to the non-uniformity profile of the mask. The particle beam can be an ion beam for use in an ion beam etching process, a reactive ion beam for use in a reactive ion beam etching process, an ion beam for use in a chemically assisted ion beam etching process, or a beam for use in a sputtering process.
Preferably, the system also comprises a mechanism of mapping the thickness non-uniformity profile of the device surface prior to adjusting the thickness. Preferably, the mapping mechanism comprises a frequency measurement device for measuring the frequency at different locations of the device surface. It is also possible to use a thickness measurement device to determine the amount of material to be removed at different locations.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 2 to 12.